DESCRIPTION (Applicant's Description): Erythropoietin-Receptor (EPO-R) signaling is crucial for differentiation, proliferation and survival of mature erythroid progenitor cells. Dysfunction of EPO-R signaling results in malignancies and other disorders of erythroid cells such as erythroleukemias and polycythemia vera. Studies during the past several years have emphasized that signaling pathways activated by growth factor receptors and tyrosine kinase receptors are redundant and shared by many receptors. They have also pointed out the absence of understanding of receptor signaling in their primary physiological setting. The overall objective of this proposal is to overcome this lack of information regarding mechanisms that regulate EPO-R signaling during normal development. The studies outlined in the present proposal are aimed to investigate the involvement and the role of MAP-kinase pathways (ERK, JNK, p38) during erythroid development using retroviral gene transfer into fetal liver cells from EPO-R knock-out mice. These mice die in embryo from severe anemia, due to an absence of differentiation of late erythroid progenitor cells. The applicant will investigate whether the potential activation of JNK and p38 (ERK is known to be activated by the EPO-R engagement) in response to the EPO-R engagement is important for cellular outcomes such as proliferation, differentiation or survival of erythroid cells. She is also cloning novel Cytokine-Inducible-SH2-containing(CIS)-Like genes which may negatively regulate EPO-R signaling. In addition, she will investigate the involvement of the negative regulators of EPO-R signaling in erythroid malignancies such as polycythemia vera. In a related project she will investigate the in vivo role of tyrosine phosphorylation of the EPO-R. In this process she will establish an in vivo chimeric animal model system using fetal liver cells from E P O-R-/- mice retrovirally transduced with specific EPO-R mutants to reconstitute erythropoiesis in lethally irradiated adult mice. Molecular understanding of EPO-R signaling and its mechanisms of dysregulation in erythroid blood disorders and malignancies is essential for development of novel therapeutic approaches such as the ones that correct the defect by gene therapy.